The present invention generally relates to a method for cleaning a silicon-based substrate in an ammonia-containing solution and more particularly, relates to a method for cleaning a silicon wafer in a cleaning solution containing ammonia without incurring ammonia vapor damages to the silicon surface of the wafer.
In the manufacture of semiconductor devices, a large quantity of deionized (DI) water is required to process silicon wafers. The consumption of DI water increases with the size of the wafers. For instance, the consumption at least doubles in the processing of 200 mm size wafers when compared to the consumption in the processing of 150 mm size wafers. DI water is most frequently used in tanks and scrubbers for the frequent cleaning and rinsing of wafers in process. It is desirable that the surface of a wafer be cleaned by DI water after any process has been conducted on the wafer, i.e., oxide deposition, nitride deposition, SOG deposition or any other deposition or etching process. Such wafer cleaning step is accomplished by equipment that are installed either in-line or in a batch-type process.
For instance, a cassette-to-cassette wafer scrubbing system is one of the most used production systems for wafer cleaning prior to either a photoresist coating, oxidation, diffusion, metalization or CVD process. A typical automated wafer scrubber combines brush and solution scrubbing by DI water. The scrubber utilizes a hyperbolic high pressure spray of DI water with a retractable cleaning brush. A typical mechanical scrubbing process consists of rotating a brush near a wafer surface that is sprayed with a jet of high pressure DI water at a pressure between about 2,000 and about 3,000 psi. The brush does not actually contact the wafer surface, instead, an aquaplane is formed across the wafer surface which transfers momentum to the DI water. The movement of the DI water thus displaces and dislodges contaminating particles that have been deposited on the wafer surface. Contaminating particles are thus removed by a momentum transfer process. As a result, larger particles become more difficult to dislodge and remove from a wafer surface.
A typical wafer scrubbing process consists of a DI water spray step followed by a spin dry and nitrogen gas blow dry step. In a typical wafer scrubbing equipment, production rates are generally between 60 to 120 wafers per hour depending on the program length. The spinning speed of the wafer is between 500 to 10,000 rpm while under a water pressure of up to 6,000 psi.
In more recently developed wafer scrubbing systems, inline systems are used which provide high pressure DI water scrubbing only while eliminating the possibility of wafer contamination by overloaded brushes. The water pressure in these systems range between 3,000 to 6,000 psi which are ejected from a nozzle mounted on an oscillating head. The wafer is spun when the oscillating spray is directed onto the wafer surface. After the cleaning step, wafer is dried by a pure nitrogen gas purge to promote rapid drying. After the scrubbing operation, wafers can be loaded into an in-line dehydration baking system for thorough drying. Batch-type systems are also used with DI water for cleaning, rinsing and drying prior to many IC processes. The systems can be programmed wherein wafers are loaded in cassettes before each cycle. One disadvantage of the batch system is their inability to be integrated into part of an automated wafer processing line.
In the conventional DI water cleaning systems, the basic requirements for the DI water cleaning system are that it provides a continuous supply of ultra-clean water with very low ionic content. It is believed that ionic contaminants in water, such as sodium, iron or copper when deposited onto a wafer surface can cause device degradation or failure. It is therefore desirable to eliminate all such ionic content from a DI water supply prior to using the water for cleaning wafers. A conventional method of measuring the ionic content in DI water is by monitoring the water resistivity. A water resistivity of 18xc3x97106 Ohm-cm or higher indicates a low ionic content in the DI water. In a conventional water purifying system, several sections which include charcoal filters, electrodialysis units and a number of resin units to demineralize the water are used for purifying the water.
Deionized water is frequently used in a wet bench process after a metal etching process has been conducted on a semiconductor wafer. When residual etchant chemical must be removed, deionized water rinse is used in a wet bench process for semiconductor wafer processing to perform two major functions of a quick dump rinse (QDR) and a cascade overflow rinse. Conventionally, the two functions are carried out in separate tanks in order to produce the desirable result. One of the major processing issues presented by the conventional dual-tank process is the particle re-deposition problem during a withdrawal step when cassettes are transported from a quick dump rinse tank to a cascade overflow tank. A second major issue is the large floor space required for accommodating the two tanks.
A conventional wet bench wafer cleaning process is shown in FIG. 1. The wet bench wafer cleaning process 10 for cleaning wafer 12 is carried out in six separate cleaning and rinsing tanks sequentially of a HF cleaning tank 14, a first quick dump rinse (QDR) tank 16, a SC-1 cleaning tank 18, a second quick dump rinse tank 20, a SC-2 cleaning tank 22 and a third quick dump rinse tank 24. The first HF cleaning tank is used to hold a diluted HF solution, for instance, at a concentration of 0.5% HF in H2O for removing a thin native oxide layer from the wafer surface. After the diluted HF cleaning process, the wafer 12 is rinsed in a first quick dump rinse tank 16 with deionized water. Wafer 12 is then cleaned in a second cleaning tank filled with SC-1 cleaning solution, i.e. a mixture of NH4OH, H2O2 and DI water at a ratio of 1:1:5. The SC-1 cleaning solution is used at a temperature between 70xcx9c80xc2x0 C. for a suitable time period. The wafer 12 is then rinsed again in a second quick dump rinse tank 20 that is filled with DI water. In the final stage of cleaning, the wafer 12 is cleaned in tank 22 filled with a cleaning solution of SC-2 which is a mixture of HCl, H2O2 and DI water at a ratio of 1:1:5. The wafer 12 is then rinsed in a third quick dump rinse tank 24 with DI water.
The wet bench wafer cleaning process 10 shown in FIG. 1 is conventionally used for pre-diffusion clean, pre-gate oxidation clean, pre-CVD clean, etc. For instance, in the ULSI fabrication of integrated devices, the conventional wet bench wafer cleaning process 10 can be advantageously used for wafer surface cleaning before a coating process in a CVD chamber or an oxidation process in a furnace.
Despite the fact that the conventional wet bench wafer cleaning process 10 is widely used, serious processing disadvantages of the process has been observed. For instance, during the SC-1 cleaning process carried out in tank 18, since SC-1 contains about 28% NH4OH which tends to form NH4OH vapor in the tank cavity over the surface of the solution. When wafer 12 is removed from the SC-1 cleaning solution and taken out of tank 18, NH4OH vapor attacks the clean wafer surface, i.e. the fresh silicon surface of the wafer. As a consequence, a defect known as xe2x80x9csilicon holexe2x80x9d frequently occurs wherein craters in the silicon surface are formed due to NH4OH vapor attack.
It is therefore an object of the present invention to provide a method for cleaning a silicon-based substrate in a wet bench cleaning process without the drawbacks or shortcomings of the conventional wet bench process.
It is another object of the present invention to provide a method for cleaning a silicon-based substrate in a wet bench cleaning process in which NH4OH vapor attack on the silicon surface is avoided.
It is a further object of the present invention to provide a method for cleaning a silicon-based substrate in a wet bench cleaning process in which an ammonia-containing solution is utilized without producing the silicon hole defect.
It is another further object of the present invention to provide a method for cleaning a silicon-based substrate in a wet bench cleaning process by first oxidizing the silicon surface before exposing the surface to an ammonia-containing cleaning solution.
It is still another object of the present invention to provide a method for cleaning a silicon-based substrate in a wet bench cleaning process by first exposing the silicon surface to an oxidant of H2O2 or O3 before exposing to SC-1 cleaning solution.
It is yet another object of the present invention to provide a method for cleaning a silicon-based substrate in a wet bench cleaning process by first exposing the silicon surface to a quick dump rinse that contains an oxidant such that a thin layer of silicon oxide of less than 10 xc3x85 thickness is formed before exposing the substrate to other cleaning solutions.
It is still another further object of the present invention to provide an apparatus for cleaning a silicon-based substrate in an ammonia-containing solution that includes a first rinse tank filled with DI water, a second rinse tank containing an oxidant for forming a silicon oxide layer on the substrate, and a cleaning tank for cleaning the substrate in an ammonia-containing solution.
In accordance with the present invention, a method for cleaning a silicon-based substrate in an ammonia-containing solution without the formation of silicon hole defects, and an apparatus for cleaning a silicon-based substrate in an ammonia-containing solution without NH4OH vapor damage are provided.
In a preferred embodiment, a method for cleaning a silicon-based substrate in an ammonia-containing solution can be carried out by the operating steps of first providing a silicon-based substrate that has a silicon surface thereon; forming a silicon oxide layer on the silicon surface; and cleaning the silicon-based substrate in an ammonia-cleaning solution.
The method for cleaning a silicon-based substrate in an ammonia-containing solution may further include the step of forming the silicon oxide layer by exposing the silicon surface to an oxidant of H2O2 or O3. The method may further include the step of forming the oxide layer to a thickness of not more than 10 xc3x85, or the step of removing the silicon oxide from the silicon-based substrate after the ammonia-containing solution cleaning step. The method may further include the step of forming the silicon oxide by exposing the silicon surface to deionized water that contains an oxidant. The ammonia-containing solution may include NH4OH. The method may further include the step of pre-cleaning the silicon surface in a pre-cleaning step utilizing HF.
The present invention is further directed to a method for cleaning a silicon-based substrate in an ammonia-containing solution which can be carried out by the operating steps of first providing a silicon-based substrate that has a silicon surface thereon; exposing the silicon surface to a quick dump rinse solution containing an oxidant; forming a silicon oxide layer on the silicon surface; and cleaning the silicon-based substrate in an ammonia-containing solution.
The method for cleaning a silicon-based substrate in an ammonia-containing solution may further include the step of forming the silicon oxide layer by exposing the silicon surface to a quick dump rinse solution containing H2O2 or O3, or the step of forming the silicon oxide layer to a thickness not more than 10 xc3x85. The method may further include the step of removing the silicon oxide layer after the cleaning step. The ammonia-containing solution may be a solution that includes NH4OH. The method may further include the step of pre-cleaning the silicon surface in a pre-cleaning process utilizing diluted HF.
The present invention is further directed to an apparatus for cleaning a silicon-based substrate in an ammonia-containing solution which includes a first rinse tank for pre-rinsing a silicon-based substrate that has a silicon surface thereon; a second rinse tank containing an oxidant therein for forming a silicon oxide layer on the silicon surface; and a cleaning tank for cleaning the silicon-based substrate in an ammonia-containing solution.
The apparatus for cleaning a silicon-based substrate in an ammonia-containing solution may further include a cleaning tank that contains HF positioned after the ammonia-containing cleaning tank for removing the silicon oxide layer from the silicon-based substrate. The second rinse tank may contain a rinse solution that includes an oxidant of H2O2 or O3. The silicon oxide layer may be formed to a thickness of not more than 10 xc3x85. The ammonia-containing solution may include NH4OH. The apparatus may further include a pre-cleaning tank that contains HF for pre-cleaning the silicon surface prior to the pre-rinsing step in the first rinse tank. The second rinse tank may be a quick dump rinse tank utilizing deionized water.